12 October 2019 | New Scientist | 39shot apart faster than the speed of light,
never to come into contact again. In some
models of this process, tiny fluctuations in
the universe’s density prior to inflation would
have resulted in small areas of extreme density
immediately afterwards. In those moments,
a fraction of a second after the big bang, these
areas could have pulled in beams of ambient
light before collapsing into black holes. Any
that formed from this primeval radiation
would rank as primordial black holes.
In 1975, the possible existence of these
cosmic old-timers sparked widespread
interest when Stephen Hawking published
a landmark paper on the properties of black
holes. He calculated that black holes should
emit particles in a process called Hawking
radiation, causing them to slowly shrink
and – eventually – evaporate.
Hawking radiation would be a particularly
significant factor for primordial black holes.
Regular, or astrophysical, black holes form
from the collapse of dense stars, with a
minimum mass of about 1.4 times that of
our sun. But primordial black holes are created
via the direct collapse of lots of radiation, so
they can be as small as you like. That is bad
news for their longevity. “How long a black
hole lives depends on its mass: the smaller it
is, the shorter it lives,” says Francesca Vidotto
at the University of the Basque Country in
Spain. Accelerated evaporation of primordial
black holes makes them an obvious place to
look for traces of Hawking radiation.
More than 40 years on, no such traces have
been spotted. And as primordial black holes
remained equally invisible, interest in them
waned. Lately, though, that is changing. “The
really exciting thing about primordial black
holes is that there are so many mysteries that
in principle they could explain,” says Bernard
Carr at Queen Mary, University of London,
who worked on these objects with Hawking.Cosmic clean-up
For one thing, they might make up the
mysterious substance we call dark matter. For
decades, galaxies have been observed rotating
faster than they should given all the visible
stuff within them. That has led cosmologists
to believe that an invisible “dark” matter
lurks within these galaxies too, giving them
the gravitational heft they need to spin at the
speeds we see without flying apart. “We’re
pretty sure dark matter exists, but we have
no idea what it is,” says Anne Green at the
University of Nottingham in the UK.
The preferred candidate has long beenI
N THE beginning, there was light. Then,
perhaps, a point of darkness. More dark
spots appeared, the light circling them
before falling in like water down a drain.
These would have been our universe’s
first inhabitants, strange baby black holes
gorging on the radiation that flooded out
of the big bang. As the cosmos expanded
and cooled, their feasting slowed.
Millions of years passed, some of the
radiation that filled the cosmos giving way to
matter, which eventually clumped together to
form the first stars, planets and galaxies. Over
time, some stars grew so large that when they
ran out of fuel and collapsed, they turned into
black holes themselves. But what happened to
their distant ancestors from the dawn of time?
Maybe those very first, primordial black holes
faded away or perhaps they were big enough
to survive to the present. Either way, they
could help solve some of the biggest problems
in cosmology. If they were ever there.
The concept of black holes, objects so
enormously dense that not even light can
escape their gravitational pull, has haunted
cosmology for decades. Until recently, we had
no direct evidence they existed. That changed
in 2015, when the Laser Interferometer
Gravitational-Wave Observatory (LIGO)
detected the aftershock of a pair of black
holes colliding 1.3 billion light years away.
Experiments such as LIGO could be our
best shot at finding evidence of primordial
black holes too. In fact, some people think we
have already spotted them. That would be a
monumental discovery because these cosmic
ancients wouldn’t only be our universe’s first
black holes, but also its most interesting.
The universe’s earliest moments saw a
period of rapid expansion called inflation.
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